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Full text of "The manufacture and properties of iron and steel"

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HEAT  TREATMENT.
307.
ture, and long enough from this point to the cold state to prevent the setting up of strains from too rapid cooling.
Practically, however, it seems to be necessary to heat consider- ; ably above the lowest critical temperature in order to insure the . thorough breaking up of the cell walls to allow the enveloping form. to permeate the grain.    This arises from the fact that the changes-: by which f errite is formed attain their maximum effect only when,. the metal is .subjected to a range of temperature which includes the . three critical points.    When steel cools slowly a certain amount of |. f errite forms at the upper point, Ar8, an additional amount at thej second point, Ar2, while the principal change occurs at the lowest v point, Arr    Thus if the metal be considered as a solid solution, it may be said that crystallization takes place at the upper point, the . solution of martensite becoming, more concentrated.   When the steel is heated, as in the case of annealing, the reverse phenomenon-takes place, for at the lowest point the grain is broken up, the pearl-ite becoming martensite, somewhat diluted by the portion of f errite which it takes up.,  If now the piece be cooled slowly without further heating, the resulting structure will be quite different from the original.    The size of the grains will be much smaller and the piece will therefore be in much better physical condition, but there will still.remain room for improvement, for throughout the mass, will be found a certain proportion of f errite, corresponding to the amount which^ ,as already explained, is transformed at the higher, temperatures of Ar2 and Ars.      *                    '
In order therefore to thoroughly disseminate the f errite and encourage to the greatest extent the formation of martensite,; it'is, necessary, to heat'to the upper critical point Ac3., This high tern-.", perature, however, gives rise to a somewhat larger grain than if the lower critical point, Aci3 had not been exceeded, so that while there, is a gain in the extent of,the transformation, the grain of the, resulting steel is coarser and there is consequently a loss in strength.. The bes't result is obtained by combining the two methods, the steel being fifst heated to the upper critical point, Ac3, and allowed to, cool slowly, by which complete transformation is effected, and then reheated just above the lower critical point, Aca, by which the grain is rendered fine and all strains obliterated. In case two heatings are out of the.question, it is generally better to heat to the upper critical point, as it is -preferable to have a slightly larger grain with a fine division of the microscopic forms, than to have a piece